Astronomy and Astrophysics
Aims: We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations.
Methods: The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components.
Results: The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3σ. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ'min=2×104 to γ'max=6×105. Light curves and spectral energy distributions data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/655/A89
The MAGIC Collaboration is integrated by 20 research institutes and university departments from Armenia, Bulgaria, Finland, Germany, Italy, Poland, Spain, Switzerland and USA. The collaboration comprises two 17m diameter telescopes, located at the Roque de los Muchachos Observatory, designed to measure the Cherenkov radiation associated with
Active Galactic Nuclei (AGN) are characterized by a strong emission coming from a very compact region (only few pcs) at the galaxy center. Blazars form a class of AGN, characterized by high luminosity in a broad frequency range, from radiofrequencies to high energies (X-rays and γ-rays), as well as extreme variability and high polarization at